Superabrasives such as diamond and cBN are commonly used in cutting, grinding, drilling, sawing and polishing applications. One of the methods to produce tools containing cBN for the above applications is brazing. However, in brazing processes, it is difficult to achieve adequate bonding between the cBN and the braze material due to the presence of boric oxide layer on the surface of the cBN which hinders the braze material wetting the surface of the cBN when the braze is melted. This lack of wetting typically results in poor adhesion of the cBN in the braze. In addition, the boric oxide layer is poorly chemically bonded to the cBN which means the interfacial bonding between the boric oxide layer and the cBN is poor. In the tool making industry both these factors has limited the uptake of cBN.
Coatings have been applied to superabrasives in order to improve the performance of superabrasive impregnated metal matrix tools. WO2005078041 (Egan et al., E6, 2005) describes a coating with a primary layer of TiN and a secondary layer of W on cBN in grit sizes from 0.1 um to 10 mm. The TiN and W combination of coatings is useful where the TiN coating would be reacted away by a constituent of the matrix material during sintering. For example, the TiN and W combination is useful where the titanium based coating would be reacted away by liquid phases used to fix a superabrasive component to another metallic or ceramic material.
However, a problem with WO2005078041 (Egan et al., E6, 2005) and other coatings on cBN is that the production of tools a non oxidizing environment is required in order to prevent damage to the coating. The oxidation of the coating surface in an oxidising environment can affect the retention of the coating in the matrix. In addition, any protection offered by the coating may be compromised. Both these occurrences restrict the benefits offered by coatings when brazing in an oxidising environment.
Therefore, brazing is normally undertaken under vacuum with inert gases and very specific brazes, all of which make the brazing process relatively difficult and expensive. Therefore, having a cBN product which could be brazed in air (an oxidizing environment) using relatively inexpensive standard brazes would simplify the brazing process and reduce costs.
In U.S. Pat. No. 5,647,878 (Iacovangelo et al., GE, 1997) and U.S. Pat. No. 5,500,248 (Iacovangelo et al., GE, 1996), the authors describe overcoming the above problems on Chemical Vapour Deposition (CVD) diamond inserts by applying a dual layer coating consisting of a WTi bonding layer and a protective braze compatible overcoat such as Ag. It is shown that the dual-coated diamond insert may be air brazed to a tool substrate in a manufacturing environment using a standard braze without a vacuum furnace or special atmosphere.
In both '878 and '248, the invention is aimed at diamond tool inserts. The substrate mentioned is primarily concentrated on CVD diamond, and all examples use CVD diamond.
EP 0 716 159 describes a CBN compact which is coated with a coating of an inner layer of WTi and an outer layer of Ag or a coating of an inner layer of Cr and an outer layer of NiCr. These two-layer coated CBN compacts were evaluated for brazability and adhesion.